Femtosecond Laser Probes Plasma Formation

Researchers at Univer-sité Claude Bernard Lyon 1 in Villeurbanne, France, and the US Army Research Laboratory in Adelphi, Md., have used the technique to investigate laser-induced breakdown in water droplets, which may have an impact on lidar studies of atmospheric pollutants and other aerosols.

Using a femtosecond pump-probe technique, researchers have investigated the dynamics of laser-induced breakdown in water droplets. The broadband light from the plasma is emitted along the laser axis, including in the backward direction, making the phenomenon useful for remote sensing of aerosols (top). A distribution of the laser intensity in the equatorial plane of a droplet illustrates the focusing effect (right). Because the plasma formation is a nonlinear process -- five photons at 810 nm are required to ionize a water molecule -- the size of the resulting plasma is smaller than the intensity distribution shown. Courtesy of Véronique Boutou, Université Claude Bernard Lyon 1.
Véronique Boutou, a senior research scientist at the university's Laboratoire de Spectrométrie Ionique et Moléculaire, explained that the team's earlier work revealed that illuminating microdroplets of water with femtosecond laser pulses produced a white-light emission suggestive of a plasma at the internal focus of the droplets. "Our first experiments were based on a couple of indirect observations: the spectrum [of the emission], which exhibits atomic lines, for example, when NaCl is added to water; and the fairly anisotropic angular distribution with a strong backward enhancement in a very narrow lobe specific of the tightness of the plasma volume," she said.

To observe the plasma formation directly, the researchers turned to a pump-probe setup. A Kerr-lens mode-locked Ti:sapphire laser oscillator seeded a chirped-pulse amplifier to produce 120-fs, 600-µJ pump pulses at a wavelength of 810 nm. Frequency doubling in BBO yielded 60-µJ, 405-nm probe pulses. A dichroic mirror separated the pulses, and a motorized delay line controlled the time delay between them by up to 4 ps to enable the researchers to investigate the dynamics of the process.

The scientists focused the pump pulses into a stream of 50-µm-diameter water droplets from a capillary tube that was squeezed by rings of piezoelectric ceramic, with the drip rate of the nozzle synchronized to the 20-Hz repetition rate of the laser so that one pulse excited one droplet and ionized the target with an estimated 1014 W/cm2 at the focus. By analyzing how the plasma scattered the light from the probe pulses at different delay times with an intensified CCD camera, the researchers determined how quickly the plasma formed, its duration, its size and how it changed in the femtoseconds following its creation.

Atmospheric sensing

They discovered that the angular distribution of the scattered light depends on the intensity of the incident pulse, which determines the volume of the plasma, Boutou said. At an intensity of 1012 W/cm2, a backscattered peak appears at approximately 150°, which has implications for remote-sensing appli-cations, such as the identification of pollutants in the atmosphere.

"Sending femtosecond pulses on water droplets yields white-light generation, and this white light is extremely directed in the backward direction," she said. "This is then exactly what you need to measure the composition of aerosols remotely using a nonlinear lidar setup."

Members of the team are affiliated with the joint French/German Teramobile (mobile terawatt) lidar project, she said. They are considering using the Ti:sapphire laser to perform similar experiments in the field.